Long term, carrying an ICE around in an electric car will make about as much sense as an ICE vehicle always pulling a trailer with a couple of horses "just in case". Manufacturers are not likely to put a lot of time and money into vehicle technology that will have a relatively short shelf life. As battery tech improves, costs drop, and the charging infrastructure fleshes out, fewer and fewer people will be interested in a PHEV over a BEV. PHEV costs are not going to fall anywhere near as fast or as far as BEV vehicles as ICE technology is not getting cheaper, it is getting more complex and expensive as they try and wring additional efficiencies and lower emissions for a mature technology. If a PHEV with 100 mile electric range had a $5K+ premium over a 400+ mile BEV, what do you think the market share would be? I'm not saying that nobody would want one or that it would not be a good choice for some, only that the market share will be a bit of a niche that will require the consumer to pony up more $$ than a pure electric.
And as to skateboard vs under the seat batteries, the Model S, Model 3, and Taycan are shorter than the Clarity and can be fitted with over 5x the batteries as the Clarity PHEV. The handicap for a PHEV is the need for the gas tank, exhaust system, etc that need to reside in the space that makes the most sense to locate batteries. The skateboard design also brings other benefits to the party - safety and handling characteristics are a couple.